Concepts

PiezoMEMS

Summary
In this paper, we report the development of a low frequency (<100Hz) resonant piezo MEMS structures for energy harvesting. The structures were fabricated on platinized silicon wafer and bulk piezo material. Pb(Zr,Ti)O3 (PZT) piezoelectric thin film was rf sputtered on platinized silicon. PZT thin film properties were optimized for energy harvesting by improving the texture degree with respect to the underlying platinized Si substrate. The silicon MEMS fabrication process flow consisted of two modules – a) the mechanical module wherein the Si cantilever structures were created and b) the electrical module for PZT capacitor formation. This simplified process flow reduces the number of unit operations by ~40%. Using this modular approach, we studied different cantilever structural designs and based on this learning, conceived a low frequency structure that resembles a circular labyrinth. We describe various methodologies for modulation of the natural frequency of a cantilever by varying its cross section dimensions along three axes. For electrical testing under vibration, a wafer level tester was engineered using gold plated pogo pins on a custom printed circuit board. As an alternative to cleanroom based Simicromachining processes, circular labyrinth structures were also manufactured from Piezo sheets, unimorphs and bimorphs using a novel non cleanroom micromachining technique called microwater jet cutting. This structure has demonstrated <100Hz resonant operation without a tip mass.

Description
We have demonstrated the first tip mass free <100Hz resonant energy harvesting device by using a circular labyrinth vibration structure. We have utilized both Si and Piezo micromachining to fabricate these structures. We achieved an average of 1.15, 6.35 and 35.89 μW for Piezo sheet, unimorph and bimorph respectively. The exceptional energy harvesting performance of these structures was realized at very a low acceleration of 0.1g. 5 turn structure provides lower frequency and requires higher matching load. We describe various techniques for tuning the resonant frequency of MEMS cantilevers using variable cross sections in the x-y and z dimensions. We detail a novel approach to the Si wafer based Piezo MEMS fabrication process. A wafer level testing methodology has been developed for vibration MEMS testing. Finally, we explain different techniques utilized to obtain the harvester constituent RF sputtered PZT thin films.

MEMFC

Summary
This paper describes the fabrication and performance results of a magnetoelectric macro fiber compos- ite (ME MFC). The magnetoelectric composite was fabricated by bonding a magnetostrictive layer to a piezoelectric layer using a novel approach of low temperature transient liquid phase (LTTLP) bonding. The composite was diced into 150 micron wide fibers and bonded to a custom designed copper flexible circuit using a spin coated low viscosity room temperature curing epoxy. ME MFC’s with varying ferrite thicknesses of 0.6 mm and 0.5 mm were fabricated and characterized for energy harvesting. The com- posite with 0.6 mm ferrite thickness achieved an open circuit voltage of 101 mV (ME voltage coefficient of 6740 mV/cmOe) and peak power of 3.1 nW across 356 k matching load at 264 Hz.

Description
We developed a low temperature jet vapor solder bonded (<125 ◦ C) magnetoelectric composite fibers and incorporated them with kapton based copper flexible circuit using a room temperature curing epoxy. With this bonding approach, we achieved an ME volt- age coefficient of 6740 mV/cmOe and 6256 mV/cmOe at 264 Hz and 259 Hz for 0.6 mm and 0.5 mm ferrite based MFC’s respectively. The resulting magnetoelectric macro fiber composites provided both low frequency vibration and magnetic energy harvesting and AC magnetic field sensing capabilities. As an AC magnetic field sensor, the sensitivity of these MFC’s was 1011 mV/mT and 938 mV/mT respectively for the two ferrite thicknesses of 0.6 mm and 0.5 mm. We also explain the various approaches to optimize the power gen- eration capability of these magnetoelectric macro fiber composites. In this paper, we thus report simultaneously the use of a new bond- ing technique for magnetoelectric composites and the fabrication and operation of the first magnetoelectric macro fiber composite. Such flexible ME MFC’s can be utilized as magnetic field actuators and sensors over curved surfaces.